Three things are required for a fire. Fuel-something that will burn in the presence of heat when combined with oxygen, thereby releasing more heat and, as a result, reducing itself to other chemical compounds. Heat-which can be considered the catalyst that accelerates the combining of oxygen with fuel, in turn releasing more heat. Oxygen--the element which combines chemically with other substances through the process of oxidation. Rapid oxidation, accompanied by a noticeable release of heat and light, is called combustion or burning. Remove any one of these elements and the fire goes out.
The National Fire Protection Association has classified fires into three basic types:
Class A Fires--Fires and ordinary combustible materials such as wood, cloth, paper, upholstery materials, etc.
Class B Fires--Fires and flammable petroleum products or other flammable or combustible liquids, greases, solvents, paints, etc.
Class C Fires--Fires involving energized electrical equipment where the electrical nonconductivity of the extinguishing media is of importance. In most cases where electrical equipment is deenergized, extinguishers suitable for use on Class A or B fires may be employed effectively.
A fourth class of fire, Class D fire, is defined as fire in flammable metal. Class D fires are not considered a basic type of fire since they are generally caused by a Class A, B or C fire. Usually, Class D fires involve magnesium.
It is known that each material, whether it be a solid, liquid, or gas, has a flame or self-ignition point. When the material is maintained at a temperature below this flame point, it will not burn. Thus, most prior methods for extinguishing fires attempt to cool the burning material below the flame point. Class A fires respond best to water or water-type extinguishers that cool the fuel below combustion temperatures. Class B and C extinguishers are effective but not equal to the wetting/cooling action of a Class A extinguisher.
Class B fires respond to carbon dioxide (CO.sub.2), halogenated hydrocarbons (halons) and dry chemicals, all of which displace the oxygen in the air and thereby make combustion impossible. Foam is effective, especially when used in large quantities. Water is ineffective on Class B fires and may cause the fire to spread.
Class C fires involving electrical wiring, equipment, or current respond best to CO.sub.2, which displaces the oxygen in the atmosphere, making combustion difficult. Dry chemicals are effective on Class C fires but have the disadvantage of contaminating the local area with powder. Also, if used on wet and energized electrical equipment, dry chemicals may aggravate electrical current leakage. Water, wet water or foam are not acceptable agents for use on electrical equipment fires.
Class D fires respond to application of dry powder, which prevents oxidation and the resulting flame. The application of water on a metal fire is to be avoided because it will cause the fire to burn more violently and can cause explosions.
The most common fire fighting agents are thus water and water based agents, dry chemical, and gas. Water and water based agents are typically used on carbonaceous fires. Water extinguishes fires by cooling the fuel below the flame point or combustion temperature. The disadvantage of water and water based agents is that water is not effective on all combustible materials. In addition, large quantities of water are not always available. While dry chemicals offer the advantage of being nonconductors of electricity, they tend to be environmentally unfriendly and hazardous to fire fighters. Gases such as carbon dioxide and halons are toxic and, thus, environmentally unfriendly.
Extinguishing Class B liquid fires presents unique difficulties to fire fighters. With respect to pools of burning liquid, combustion takes place adjacent the surface of the liquid due to evaporation of the liquid into the air. While foam has been used in the past to smother the flame on liquid fires, it has the disadvantage of requiring substantial amounts of foam material to completely extinguish the fire. Another method gaining rapid commercial acceptance utilizes a chemical that causes a film and foam layer on top of the liquid pool to interrupt evaporation. This chemical is commonly known as aqueous film-forming foam (AFFF). The disadvantages of AFFF are that it requires a special nozzle to apply the agent, the agent takes a substantial amount of time to build a foam layer sufficiently thick to extinguish the flames, and it does not lower the temperature of the liquid pool, presenting the danger of reignition when the liquid fuel is agitated. Self-propelled three-dimensional liquid fires, such as oil and gas wellhead fires, are not extinguishable using foam because the liquid is expelled into the air at a high rate from the wellhead, with the residue forming burning liquid pools on the ground. This type of fire is extremely hot and requires very large amounts of fire fighting agent to permanently extinguish the fire. One method for extinguishing oil and gas wellhead fires uses an explosive charge that is detonated immediately above the wellhead to force the flame front away from the wellhead area. This temporarily creates a vacuum that deprives the fire of oxygen. If the liquid fuel is not immediately doused and cooled with a fire fighting agent, the returning air rushing in to fill the vacuum can cause the hot liquid fuel to reignite. This particular method is extremely dangerous and also requires substantial amounts of fire fighting agent.
Consequently, there is a need for a fire extinguishing and inhibiting material that not only rapidly and permanently extinguishes a pool of burning liquid, but is also effective in extinguishing self-propelled liquid fuel fires.